This invention relates to centrifuges, both of the continuous-feed filtering or screening type and the solid-bowl type. This invention also relates to an associated method for operating a centrifuge.
Industrial centrifugation processes for separating particulate material from various impurities include sedimentation and filtration. Generally, the particulate material is produced as a cake having different degrees of moisture depending on the type of particulate material and the particular separation process. The cake constitutes a heavy phase whereas a filtrate or centrate constitutes a light phase. In some applications, a mother liquor displaced from the dewatered cake by a washing process in a centrifuge is the valuable component while the cake is the reject. In other applications, resins or crystals in the cake are the valuable product, impurities in the cake being removed with the filtrate or centrate.
A decanter-type centrifuge has a conveyor in the form of one or more helical screw wraps rotating at a slightly different angular velocity from the velocity of the bowl or outer wall. Where the bowl has a solid wall with a cylindrical shell followed by a conical shell or beach and extends from a clarifier pool at an input or feed end of the centrifuge to a cake discharge opening or openings at an output end of the centrifuge, the centrifuge is known as a decanter or a solid bowl. A sedimentation process occurs in the cylindrical portion of the centrifuge and a dewatering of the cake in the conical dry beach area. Where the bowl is provided with one or more screen sections downstream and outside of the clarifier pool, the decanter-type centrifuge is known as a screenbowl centrifuge and performs a filtration process.
Another kind of filtration centrifuge is a pusher or pusher basket. Such a centrifuge includes a first cylindrical basket at an input end of the centrifuge and a second cylindrical basket of greater diameter at a cake output end of the centrifuge. The baskets rotate at a high angular speed. In addition, the baskets of this two-stage basket system are longitudinally reciprocatable relative to one another, whereby pusher plates shove the heavy phase particulate material in a layer along the first basket, from the first basket to the second basket, and along the second basket to a cake discharge port. Single-stage pushers or pushers with two-or-more stages such as quadruple-stage pushers are also available.
Filtering centrifuges have been used to wash the cake to remove the impurities. There are two types of washing: a spray wash and a flood wash. In a spray wash, wash liquid is applied to a localized area on the cake surface in an attempt to displace mother liquor which contains the impurities. Spray washing is used most commonly in a screenbowl centrifuge where the cake height varies across the screen from a thin layer to a thick layer adjacent to the pressure face of the conveyor blade.
Another kind of centrifuge, used particularly for the dewatering and washing of thickened slurries with particulate solids, is a conical-screen centrifuge. The centrifuge wall includes a conical screen which has an increasing diameter in the cake flow direction. The particulate solids are held by the screen as the liquid filters through. The conical screen has the advantage that the cake experiences an increasing centrifugal gravitational force as the cake travels down to the large diameter of the cone. The centrifugal gravity is proportional to the radius of the screen for a given rotational speed of the basket. Another advantage of the increasing-diameter conical screen is that, for a given cake mass, the cake height and thus the resistance to liquid drainage are reduced as the cake moves towards the large-diameter end of the cone, owing to the conservation of mass. Both of these advantages enhance the dewatering of the cake. Also, spray washing is used in conical-screen centrifuges to remove impurities dissolved in the mother liquor.
In a conical-screen centrifuge, a thickened or concentrated feed is introduced, after pre-acceleration to the proper tangential speed, into the centrifuge at the smaller end of the conical screen. The cake travels down the cone when the half cone angle, typically 30.degree. to 40.degree. with respect to the axis of the machine, is steep enough to overcome frictional forces.
When the cone angle is small, typically 15.degree. to 25.degree., a mechanical conveyance mechanism is used to convey the cake from the small end of the cone to the large end thereof One mechanism is a helical screw conveyor with a single continuous lead. Another, related, mechanism is a multiple-lead screw conveyor (4 leads is common). Yet another mechanism is a set of discrete scraper blades each conforming to a helix. In any case, the conveyor rotates at a differential speed as compared to the screen, thereby conveying cake down the screen. By adjusting the differential speed, the cake movement and concomitantly the cake residence time can be adjusted. Another mechanism is a vibrator, such as rotation of eccentric weights with an axis of vibration parallel to the axis of the machine. The inertia force generated by the vibration propels the cake from the small end to the large end, the discharge end, of the centrifuge.
Pusher centrifuges are excellent for washing crystals for particles having a size greater than 75-100 microns, while screen bowls provide adequate washing when the mean particle size of the processed crystals is larger than 45 microns. For chemical applications such as in fine resins separation where particles are in the 5 to 30 micron range, both types of equipment are limited by fine solids passing through the screen. Instead, batch perforate basket centrifuges are used with a filter cloth having fine openings to prevent loss of fine solids in the filtrate. Batch processes, however, require the use of surge tanks for interim storage and introduction of the feed, which may be unacceptable in certain applications. Also, with both batch and continuous centrifugal filters, the centrifugal force is limited to a maximum of 1000-2000 g, which is inadequate for dewatering fine particles with low-permeability cake. Furthermore, the moisture trapped in the capillaries of the cake for the batch basket can be significant, especially for fine particles. This is compensated in part in the batch basket process by providing a long washing and dewatering time, with the result of a lower solids throughput.
Solid-bowl decanters have been used for washing fine resins without the disadvantage of losing the fine particles. In one application, the resin slurry after exiting a reactor is introduced into a decanter centrifuge wherein the cake is first dewatered in a dry beach area and subsequently washed with an appropriate liquid to displace the cake mother liquor (the valuable part), which flows back to the pool. The mother liquor is then discharged with the centrate. The cake (reject part) is dewatered before discharge. In another application using a solid-bowl centrifuge, the resin or crystal solids are the valuable component. By washing, the impurities in the cake are reduced before the cake is discharged from the solid-bowl centrifuge. The impurities dissolved in the wash liquid leave the machine with the centrate.
However, with the solid bowl, the washing which takes place in the dry beach after the cake has been conveyed by the screw is limited as the retention time is very short, on the order of a few seconds or less. The most important disadvantage is that the wash liquid together with the impurities or valuables in the mother liquor are conveyed out with the cake. This limits the use of solid bowls in cake washing.
FIG. 1A shows a conventional solid bowl 10 with a single beach 12 provided with spray nozzles 14 for washing of the cake 16 after the cake comes out of an annular pool 18. The washing and dewatering time is extremely limited. Also, wash liquid and displaced mother liquor may both get conveyed with the cake to discharge 20, rendering washing ineffective.
FIG. 1B shows an improvement, not believed to exist in the art, wherein washing via nozzles 14 takes place in a second beach section 22 of a compound beach 24 in which the second beach section has a shallower angle .beta..sub.2 than the angle .beta..sub.1 of a first beach section 26. Cake 16 is pushed by a helical conveyor blade 28 along a helical cake flow path of generally decreasing radius. Retention time is increased. Still, wash liquid containing either valuable product or impurities may be carried with the cake 16 to discharge 20 as the surface velocity of the cake is oriented downstream and the component of centrifugal gravity directed upstream along the cake flow path, which is responsible for returning the liquid back to the pool 18, is reduced because of the reduced beach angle and climb angle.